I realize Marschner doesn't make the role of boron very clear. I refer to Marschner's second edition on silicon, pages 417 - 426. Marschner classifies silicon as 'beneficial' rather than essential. Boron he considers essential, pages 379 - 396. Although he acknowledges their close similarity, he doesn't make the connection I do. And Marschner is, I reckon, the best of the lot as far as academia is concerned. For instance he does make a connection between manganese and silicon, which I reckon is a very important connection, (p. 423).